Skip to main content

Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

See other formats


ORDINARY  BRONZES

333

The filter and precipitate are heated with 2-3 c.c. of 10% sodium
hydroxide, and 10-15 c-c- of sodium sulphide solution (D 1-225) in a small
beaker on a water-bath until the metastannic acid is completely dissolved.
After dilution with 10-15 c-c- f water and addition of a little sodium sul-
phite dissolved in a little water to reduce polysulphides, the liquid is again
heated for a short time and filtered by decantation, the filtrate being collected
in a f-litre conical flask. The residue is again treated with 10-15 c-c- f
sodium sulphide solution, heated, reduced with sulphite, diluted, filtered and
washed with hot water containing a little sodium sulphide until about
150-200 c.c. of filtrate are collected. The lead, copper, iron, etc., contained
in the metastannic acid remain on the filter as sulphides. The filter is
dried and then burnt in a small dish, the residue being dissolved in a little
nitric acid (D 1-3) and the solution obtained added to the liquid in which
the copper, lead, zinc, etc., are to be determined.

As regards the determination of the tin and antimony, hydrochloric
acid diluted with an equal volume of water is added gradually and with
shaking (under a draught hood) to the solution in sodium sulphide until
the latter shows an acid reaction ; 25-30 c.c. of concentrated hydrochloric
acid are then added and the liquid boiled to dissolve the separated sul-
phides, adding, if necessary, a few crystals of potassium chlorate.

When cold, the solutionwhich contains sulphur and is turbidis
rendered alkaline with ammonia and then heated with 5 grams of oxalic
acid. When all the tin hydroxide precipitated by the addition of ammonia
is redissolved, 7 grams of ammonium oxalate are added, the liquid heated
to boiling and hydrogen sulphide passed for about 15 minutes through
the boiling liquid. Under these conditions, any antimony present is alone
precipitated.

After being allowed to cool somewhat in the current of hydrogen sulphide,
the liquid is filtered, the filtrate being collected in a |-litre beaker and the
washing effected with a hot i% oxalic acid solution saturated with hydrogen
sulphide. The antimony sulphide precipitate remaining on the filter is
analysed according to 2 (below).

The filtrate is treated with 30 c.c. of cone, hydrochloric acid and 20
grams of ammonium oxalate, boiled for a short time to eliminate hydrogen
sulphide, a few drops of hydrogen peroxide added, heated again for a short
time, allowed to cool to 40-50, made up to about 300 and electrolysed,
if possible under a hood, to determine the tin : coppered Winkler cathode 3 ;
spiral anode ; ND100  i amp. ; voltage = 3-4 ; temperature, 40-50,
and duration (with 0-1-0-3 gram Sn) 4-5 hours.

When the deposition of the tin is complete, the electrolytic beaker is
replaced, without interrupting the current, by another full of water ; after
a time the cathode is detached, washed with an abundant supply of water
and then with alcohol and ether, and dried at 70.

1 To prevent attack of the copper coating by the acid solution while the arrange-
ments for the electrolysis are being made, the electrodes should be fixed on supports
at the required height and connected with the source of current, the beaker being then
raised to immerse them. In this way the current begins to flow immediately the elec-
trodes dip into the liquid and so prevents attack of the copper.d from which thetreated with an excess